How to export data based on pop-up-menu choice to a .txt file?

I need to store a numerical value inside a .txt file. The value is determined based on which option in the pop-up-menu has been selected? The problem I'm having is that whenever I check the .txt file afterwards it is blank? If I remove the semi-colons then it displays the correct value for each choice, e.g. removing the semi-colon from the line a = -10; causes a = -10 to be displayed in the command window when I select the second option in the pop-up-menu? So why can't it write it onto a a text file? I've shown my code below

% --- Executes on selection change in popupmenu2.
function popupmenu2_Callback(hObject, eventdata, handles)
% hObject    handle to popupmenu2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: contents = cellstr(get(hObject,'String')) returns popupmenu2 contents as cell array
%        contents{get(hObject,'Value')} returns selected item from popupmenu2

contents = get(hObject,'Value');

switch contents 

      case 1            %Nothing selected

          a = 0;

          fid = fopen('Cstern.txt', 'wb');

          fprintf(fid, '%s\n', a);

          fclose(fid);

      case 2            %V-shaped sections         

          a = -10;

          fid = fopen('Cstern.txt', 'wb');

          fprintf(fid, '%s\n', a);

          fclose(fid);

      case 3            %U-shaped sections      

          a = 10;

          fid = fopen('Cstern.txt', 'wb');

          fprintf(fid, '%s\n', a);

          fclose(fid);

      case 4            %None of the above

          a = 0 ; 

          fid = fopen('Cstern.txt', 'wb');

          fprintf(fid, '%s\n', a);

          fclose(fid);

end

Answer 1

[Walter Roberson]

Perhaps it is writing to a Cstern.txt in a directory other than the one you are looking in. It is going to write the file into whatever directory is the current directory. I suggest you display pwd() to show you which directory that is.

I further suggest you get into the habit of defining a specific output directory and using fullfile() to qualify the file names so you know exactly where the output is going to end up. For example,

outdir = handles.output_directory;
outfile = fullfile( outdir, 'Cstern.txt');

...
[fid, message] = fopen(outfile, 'wb');
if fid < 0
  warndlg(sprintf('Failed to open file "%s" for writing because: %s', outfile, message));
  return
end

[Alfie Grace]

Hi Walter, thanks for the quick answer

It's definitely writing it in the write place, whenever an option is selected in the pop-up-menu the Cstern .txt file appears in my working directory. The problem is that when I look at it, the .txt file is blank? Do you perhaps have any more possible solutions as to what I could be doing wrong?